Methods for the formation on a wall exposed to frictional forces and belonging to a light alloy element, of a wear-resistant composite coating metallic

ABSTRACT

The coating is essentially constituted of nickel containing a dispersed phase of hard fine particles of silicon carbide. The method includes the operational phases: forming by chemical means a thin layer of zinc, then depositing electrolytically a composite layer of nickel (containing hard particles in a dispersed phase to a desired content) until the thickness desired for the whole of the coating is obtained; the wall being subjected initially to a shot-blasting treatment, and/or before any deposition of a metallic layer, attacked with a sodium compound in a bath of said compound. The electrolytic deposits of nickel (prenickeling or final deposit of nickel with a dispersed phase of hard particles) are conducted by causing the electric current to be passing into the bath already at the moment when immersion of the elements in the course of treatment is effected, due to which a passivation of the elements prejudicial to good adherence of the final coating is avoided. Transfer of the treated element from one vat to another, is effected in a time of the order of several seconds to one minute. Said sodium compound attack may be followed by a nitric-hydrofluoric attack in a concentrated bath. Before any deposition of nickel, two successive zinc treatments may be effected at ambient temperature, the layer of zinc deposited on the first zinc treatment being dissolved in concentrated nitric acid before the second zinc treatment.

United States atent [1 1 Durin [451. Feb. 25, 1975 1 METHODS son THE FORMATION ON A COMPOSITE COATING METALLIC [75] Inventor: Michel Durin, Paris, France [73] Assignee: Societe Anonyme Automobiles Citroen, Paris, France [22] Filed: Nov. 8, 1972 [21] Appl. No.: 304,842

[30] Foreign Application Priority Data Nov. 9, 1971 France 71.40153 [52] US. Cl. 204/33, 204/38 B [51] Int. Cl. C23b 5/50, C23b 5/62 [58] Field of Search 204/33, 35 R, 38 B, 49

References Cited UNITED STATES PATENTS 2,745,799 5/1956 Patrie 204/38 B 2,752,302 6/1956 Magarrus 204/29 3,041,259 6/1962 Stoddard 204/33 X 3,166,444 1/1965 Ehren et al. 204/33 X 3,235,404 2/1966 Mickelson et al. 204/33 X 3,438,789 4/1969 Weiss et a1 204/35 N X 3,493,474 2/1970 Weber 204/33 3,640,799 2/1972 Stephan 204/38 B 3,687,824 8/1972 Brown et al. 204/40 ,2QZ L l9 19l2-. ,9k' 5%!sew-t Primary Examiner-F. C. Edmundson Attorney, Agent, or Firm--Stcvens, Davis, Miller & Mosher [57] ABSTRACT The coating is essentially constituted of nickel containing a dispersed phase of hard fine particles of silicon carbide. The method includes the operational phases: forming by chemical means a thin layer of zinc, then depositing electrolytically a composite layer of nickel (containing hard particles in a dispersed phase to a desired content) until the thickness desired for the whole of the coating is obtained; the wall being subjected initially to a shot-blasting treatment, and/or before any deposition of a metallic layer, attacked with a sodium compound in a bath of said compound. The electrolytic deposits of nickel (prenickeling or final deposit of nickel with a dispersed phase of hard particles) are conducted by causing the electric current to be passing into the bath already at the moment when immersion of the elements in the course of treatment is effected, due to which a passivation of the elements prejudicial to good adherence of the final coating is avoided. Transfer of the treated element from one vat to another, iseffected in a time of the order of several seconds to one minute. Said sodium compound attack may be followed by a nitric-hydrofluoric attack in a concentrated bath. Before any deposition of nickel, two successive zinc treatments may be effected at ambient temperature, the layer of zinc deposited on the first zinc treatment being dissolved in concentrated nitric acid before the second zinc treatment.

METHODS FOR THE FORMATION ON A WALL EXPOSED TO FRICTIONAL FORCES AND BELONGING TO A LIGHT ALLOY ELEMENT, OF A WEAR-RESISTANT COMPOSITE COATING METALLIC The invention relates to-methods for the formation, on a wall exposed in service to frictional forces and belonging to an element of light alloy (especially of an aluminium alloy with a high content of silicon), ofa composite metallic coating, resistant to wear, constituted by a matrix of nickel containing a dispersed phase of particles of great hardness generally formed essentially by silicon carbide. The invention relates more particularly, because it is in this case that its application seems to offer the most advantage, but not exclusively, among these methods, to those for forming a composite metallic coating (of the above-indicated type) on the inner wall of the light alloy casing (currently called trochoidal) of a rotary piston engine.

There has already been proposed a method of this type for the coating of trochoids according to which the wall to be lined with a composite metallic coating of nickel with a dispersed phase of silicon carbide received successively a thin layer of zinc (thickness of about 1 micron) deposited 'by chemical means, then a thicker support layer of nickel (16 to 20 microns) deposited by electrolytic means and containing no'or few particles of silicon carbide, and lastly, also by electrolytic means, a layer of nickel with a dispersed phase of silicon carbide, the latter layer having a thickness such that the overall thickness of the coating is around 300 to 320 microns before final trueing.

The important problem, for a stratified coating of this type, is to obtain a good adherence of each layer on the sub-adjacent layer so that the coating proper has a monolithic character and so that the whole of said coating adheres firmly to the treated wall.

Now experiment has shown that prior methods, especially under certain conditions of use of the treated elements, only resolve in an imperfect manner the problem explained above, the composite metallic coating being then often spoiled by phenomena of detachment accompanied by fissures and cracks.

It is a particular object of the invention to overcome these drawbacks by providing a method enabling the formation of a composite metallic coating having improved structural mechanical strength and adherence to the treated wall relative to those of coatings of the same type obtained by prior methods.

' essentially constituted of nickel containing in the dispersed phase hard fine particles generally of silicon carbide, on a wall exposed in service to frictional forces and belonging to an element of light alloy (especially of aluminium alloy with a high content of silicon) which can be for example a trochoid of which the inner wall must then receive the abovesaid coating, this method comprising, among other operational phases and in manner known in itself, the formation, by chemical means of a thin layer of zinc, advantageously followed by an electrolytic treatment of prenickeling ensuring the formation of a thicker layer of nickel containing no or few hard particles in the dispersed phase,

then the electrolytic deposition of a composite layer of nickel (containing hard particles in the dispersed phase to the desired content) until the obtaining of the desired thickness for the whole of the coating, the abovesaid method being characterized in that the wall to be lined with the. coating concerned is subjected initially, that is to say before any other treatment of the operational process, to a shot-blasting treatment advantageously effected by dry projection onto the abovesaid wall, of glass microbeads projected by a current of air under a pressure preferably comprised between 5 and 10 bars for a duration preferably comprised between 10 and 20 seconds per 0.05 dm2 of treated surface.

Experiment has shown that such an initial treatment of shot-blasting mechanically prepares the wall which has to receive the coating and increases the adherence of the first metallic layer applied to this wall, which has the effect of improving the attachment of the whole of the coating on the treated element. In addition, in the case of sodium compound attack of the wall to be treated before any deposition of metallic layer (which attack will be more explicitly considered below) the abovesaid initial treatment of shot-blasting enables considerable reduction in the temperature of the bath (for example reduction from 60 to 20C) and the dura tion of attack (for example reduction from 4 minutes to 2 minutes).

The invention consists, apart from this main feature,

of certain complementary features which are preferably but not necessarily used in combination with the said principal feature.

According to one of these complementary features, the wall which has to receive the wear-resistant composite metallic coating is subjected before any deposition of metallic layer, to a sodium compound attack conducted in a sodium compound bath if necessary supplemented with a polyhydroxy acid (preferably glu conic acid) or with a salt of such an acid (preferably a sodium salt), this addition having'the effect of securing a slight increase in adherence by uniformization of the attack and through improvement in the rinsability of the elements, and of avoiding the formation of an interfering viscous compound on the surface to be coatedand on the walls ofthe tank.

Such a sodium compound attack can advantageously be conducted in a bath containing from 50 to 150 grams per liter (preferably of the order of grams per liter) of caustic soda and from 5 to 15 grams per liter (preferably of the order of 10 grams per liter) of polyhydroxy acid or of a salt of such an acid (for example sodium gluconate).

' The abovesaidbath is advantageously at ambient temperature (between 15 and 30C).

When the wall which has to receive the composite metallic coating has been subjected initially to a shotblasting treatment according to the principal feature of the invention, the sodium attack which has just been considered will take place after the abovesaid shotblasting treatment and before any deposition of a constituent metallic layer of the coating.

It is interesting to note that increases in the adherence of the coating from 30 to 50 percent have been observed by proceeding with an initial shot-blasting of the wall followed by sodium attack, these two operations being conducted under the above-indicated conditions.

According to a further feature of the invention, relating to the case where the method comprises, before any deposition ofmetallic layer, a sodium compound attack, this sodium compound attack can be followed by a nitric-hydrofluoric attack in a concentrated bath, which enables further improvement of the adherence of the coating, especially when the treated element is of light alloy rich in silicon.

According to another additional feature of the invention, the electrolytic deposits of nickel (prenickeling or final deposition of nickel with a dispersed phase of hard particles) are conducted by causing the electric current to be passing already into the bath at the moment when immersion of the elements in the course of treatment is effected, the density of the abovesaid current being preferably comprised between 1 and 3 A/dm2 during this immersion phase, due to which alteration of the surfaces to be treated prejudicial to good adherence of the final coating is avoided.

According to yet another additional feature of the invention, the adherence of the coating is further improved by subjecting the wall to be coated, before any deposition of nickel, to two successive zincing treatments effected at ambient temperature, the layerof zinc deposited on the first zincing being dissolved in concentrated nitric acid before the second zincing.

According to yet another additional feature of the invention, relating to the case where the method comprises a prenickeling phase (electrolytic deposition of nickel containing no or few hard particles in the disdiate rinsing, so as .to avoid any passivation of the abovesaid element due to remaining in air for a prolonged period, the abovesaid transfer being effected under the passage of current so as to avoid passivation by absorption oforganic products in the following bath, which passivations would in any case be prejudicial to good adherence of the final coating.

The invention relates also to light alloy elements (for example trochoids for rotary piston engines), in particular of aluminium alloy with a high silicon content, comprising a wall provided with a metallic coating based on nickel containing a dispersed phase of hard particles (silicon carbide for example), said coating being formed according to all or some of the preceding features.

There will now be shown, in a specific example, how the various features of the invention can be carried out and integrated into a complete forming process, on the inner wall of a trochoid, of a wear-resistant metallic coating essentially constituted by a matrix of nickel containing a dispersed phase of silicon carbide.

Listed below, in the order in which they are effected, are the successive operational phases of the said complete process.

After having effected a mechanical finishing by fine machining of the inner wall of a trochoid constituted for example of light alloy with 10% Si, the said wall is subjected to shot-blasting by production of glass beads of a diameter comprised between 75 and 150 microns, under an air pressure of 7 bars, for a duration of l minute per 0.7 dm2 of wall and per nozzle.

There is then effected, for two minutes and at a temperature of the order of 20 to 25C, a sodium compound attack in a bath containing grams per liter of caustic soda and 10 grams per liter of sodium gluconate.

This is followed, for two minutes and still at a temperature of the order of 20 to 25C by a nitrichydrofiuoric acid attack in a bath constituted by three parts by volume of 62 percent nitric acid and one part by volume of 40 percent hydrofluoric acid.

Then there is carried out, for 2 minutes at a tempera ture of the order of 20 to 25C, a first chemical zincing in an alkali bath, preferably containing iron, after which the layer of zinc is dissolved by immersion, for 1 minute at a temperature of the order of 20 to 25C, in an undiluted 62 percent nitric acid bath.

This is followed by a second zincing under the same conditions as those of the first zincing, but with a duration of immersion of only 25 seconds.

There is then carried out an electrolyticprenickeling for 20 to 30 minutes with a current density of 2 to 3 A/dm2 in a tank with nickel anodes containing a bath at about 40C and at pH 4 to 4.5, which bath has the following composition: nickel sulfamate (400g/l), nickel chloride (4 g/l), boric acid (40 g/l), if necessary saccharine (for example in the form of sodium saccharinate), and a wetting agent. And, lastly, the electrolytic deposition of the composite wear-resistant coating is effected in a bath at 55C, with nickel anodes, containing crystalline nickel sulfamate (400g/l), nickel chloride (4 g/l), boric acid (40 g/l), about 3 g/l of saccharine (for example in the form of a sodium saccharinate solution), particles of silicon carbide g/l), and a wetting agent. I

The immersion is carriedout under a current of l to 2 A/dm2 and it is followed by a gradual rise to 20 A/dm2 during 15 minutes, after which the deposition is pursued for one hour thirty minutes at 20 A/dm2.

For the electrolytic treatments, there may be used a material comprising the application of the features which are the subject of French patent applications filed 9 Nov. l97 l for: Device and method for the production of a coating, for example electrolytic, on the walls of members exposed in service to frictional forces and for: Improvements in installations for producing an electrolytic coating, under US. Pat. Nos. 71, 40, 151 and 71,40, 152, respectively.

I claim:

1. Method for the formation of a metallic wearresistant coating, comprising primarily nickel containing a dispersed phase of hard fine particles, on a wall to be exposed in service to frictional forces, said wall comprising a light metallic alloy, said method includ ing: forming by chemical means a thin layer of zinc on said wall, then depositing electrolytically a composite layer of nickel containing hard particles in a dispersed phase; said method being characterized in initially subjecting the bare wall to a shot-blasting treatment.

2. Method of claim 1, additionally comprising subjecting the bare wall to attack with a sodium compound conducted in a bath of said compound before deposition of any metallic layer on said wall.

3. Method according to claim 2, wherein said sodium compound attack is conducted in a bath containing 50 to 150 grams per liter of caustic soda and 5 to 15 grams per liter of a polyhydroxy acid.

4. Method according to claim 3, wherein said polyhydroxy acid or salt is present to the extent of about grams per litre.

5. Method according to claim 2, wherein said sodium compound bath is supplemented with a polyhydroxy acid or a salt thereof.

6. Method according to claim 5, wherein said polyhydroxy acid is gluconic acid.

7. Method according to claim 5, wherein said salt is the sodium salt.

8. Method according to claim 2, wherein the bath contains about 100 grams per litre of caustic soda.

9. Method according to claim 2, wherein said bath is at a temperature between l5 and 30C. I

10. Method of claim 1 said method being characterized in that the electrolytic deposits of nickel are conducted by causing the electric current to be passing into the bath already at the moment when immersion of the elements in the course of treatment is effected, due to which a passivation of the elements prejudicial to good adherence of the final coating is avoided.

11. Method according to claim 10, wherein said current density is between 1 and 3 A/dm during the immersion phase.

12. Method according to claim 1, additionally comprising a prenickeling phase comprising an electrolytic deposition of nickel containing no or little hard particles in the dispersed phase, followed by electrolytic deposition of nickel containing hard particles in a dispersed phase, wherein transfer of the treated element from one vat to another is effected in a time of the order of several seconds to one minute.

13. Method according to claim 12, wherein said transfer is effected without intermediate rinsing.

14. Method according to claim 1, wherein said light alloy is an aluminium alloy with a high content of silicon. I

15. Method according to claim 14, wherein said element is a trochoid whose inner wall has to receive the metallic wear-resistant coating.

16. Method according to claim 1, wherein said shotblasting treatment is effected by dry projection of glass microbeads by a current of pressurised air onto the wall.

17. Method according to claim 16, wherein the air pressure is from 5 to 10 bars.

18. Method according to claim 17, wherein the duration of said shot-blasting is from 10 to 20 seconds per 0.05 dm of treated surface.

19. Method of claim 16, wherein the glass micro beads have a diameter in the range of to microns.

20. Method of claim 1, wherein the wall to be lined with the wear-resistant coating is subjected, before any deposition of nickel, to two successive zinc treatments effected at ambient temperature, the layer of zinc deposited on the first zinc treatment being dissolved in concentrated nitric acid before the second zinc treatment.

21. Method according to claim 1, wherein said hard fine particles are of silicon carbide.

22. Method according to claim 1, wherein the forming of the thin layer of zinc is followed by an electrolytic treatment of pre-nickeling ensuring the formation of a thicker layer of nickel containing no or very few hard particles in the disperse phase. 

1. METHOD FOR THE FORMATION OF METALLIC WEAR-RESISTANT COATING, COMPRISING PRIMARILY NICKEL CONTAINING A DISPERSED PHASE OF HARD FINE PARTICLES, ON A WALL TO BE EXPOSED IN SERVICE TO FRICTIONAL FORCES, SAID WALL COMPRISING A LIGHT METALLIC ALLO Y, SAID METHOD INCLUDING: FORMING BY CHEMICAL MEANS A THIN LAYER OF ZINC ON SAID WALL, THEN DEPOSITING ELECTROLYTICALLY A COMPOSITE LAYER OF NICKEL CONTAINING HARD PARTICLES IN A DISPERSED PHASE, SAID METHOD BEING CHARACTERIZED IN INITALLY SUBJECTING THE BARE WALL TO A SHOT-BLASTING TREATMENT.
 2. Method of claim 1, additionally comprising subjecting the bare wall to attack with a sodium compound conducted in a bath of said compOund before deposition of any metallic layer on said wall.
 3. Method according to claim 2, wherein said sodium compound attack is conducted in a bath containing 50 to 150 grams per liter of caustic soda and 5 to 15 grams per liter of a polyhydroxy acid.
 4. Method according to claim 3, wherein said polyhydroxy acid or salt is present to the extent of about 10 grams per litre.
 5. Method according to claim 2, wherein said sodium compound bath is supplemented with a polyhydroxy acid or a salt thereof.
 6. Method according to claim 5, wherein said polyhydroxy acid is gluconic acid.
 7. Method according to claim 5, wherein said salt is the sodium salt.
 8. Method according to claim 2, wherein the bath contains about 100 grams per litre of caustic soda.
 9. Method according to claim 2, wherein said bath is at a temperature between 15* and 30*C.
 10. Method of claim 1 said method being characterized in that the electrolytic deposits of nickel are conducted by causing the electric current to be passing into the bath already at the moment when immersion of the elements in the course of treatment is effected, due to which a passivation of the elements prejudicial to good adherence of the final coating is avoided.
 11. Method according to claim 10, wherein said current density is between 1 and 3 A/dm2 during the immersion phase.
 12. Method according to claim 1, additionally comprising a prenickeling phase comprising an electrolytic deposition of nickel containing no or little hard particles in the dispersed phase, followed by electrolytic deposition of nickel containing hard particles in a dispersed phase, wherein transfer of the treated element from one vat to another is effected in a time of the order of several seconds to one minute.
 13. Method according to claim 12, wherein said transfer is effected without intermediate rinsing.
 14. Method according to claim 1, wherein said light alloy is an aluminium alloy with a high content of silicon.
 15. Method according to claim 14, wherein said element is a trochoid whose inner wall has to receive the metallic wear-resistant coating.
 16. Method according to claim 1, wherein said shot-blasting treatment is effected by dry projection of glass microbeads by a current of pressurised air onto the wall.
 17. Method according to claim 16, wherein the air pressure is from 5 to 10 bars.
 18. Method according to claim 17, wherein the duration of said shot-blasting is from 10 to 20 seconds per 0.05 dm2 of treated surface.
 19. Method of claim 16, wherein the glass micro beads have a diameter in the range of 75 to 150 microns.
 20. Method of claim 1, wherein the wall to be lined with the wear-resistant coating is subjected, before any deposition of nickel, to two successive zinc treatments effected at ambient temperature, the layer of zinc deposited on the first zinc treatment being dissolved in concentrated nitric acid before the second zinc treatment.
 21. Method according to claim 1, wherein said hard fine particles are of silicon carbide.
 22. Method according to claim 1, wherein the forming of the thin layer of zinc is followed by an electrolytic treatment of pre-nickeling ensuring the formation of a thicker layer of nickel containing no or very few hard particles in the disperse phase. 